1. Field of the Invention
The present invention relates to an image pickup apparatus, and particularly, to an image pickup apparatus that includes a lens apparatus and a camera apparatus including an optical element that can be attached and removed from the lens apparatus and that can be inserted and removed from an optical path.
2. Description of the Related Art
Conventionally, an image pickup apparatus is known in which refractive power is provided to an optical element inserted to an optical path to control a change in an image forming position caused by insertion and removal of the optical element.
For example, Japanese Patent Application Laid-Open No. S63-25612 discloses an invention in which an inserted optical element has refractive power to correct a change in the image forming position when the optical element is inserted. If a parallel-plate optical element is inserted between an image pickup lens and an image forming surface and into a lens system of the image pickup lens, the image forming position shifts to an over side (opposite side of an object side). Japanese Patent. Application Laid-Open No. S63-25612 provides positive refractive power to the inserted optical element to cancel a movement in the image forming position.
However, optical characteristics are changed by insertion and removal of the optical element in the conventional technique disclosed in Japanese Patent Application Laid-Open No. S63-25612. For example, if refractive power is provided to the optical element inserted to the optical path to cancel the shift in the image forming position as in Japanese Patent Application Laid-Open No. S63-25612, a spherical aberration to an under side occurs when the optical element is inserted. FIG. 17 illustrates a schematic diagram of the spherical aberration when the shift in the image forming position is canceled as in Japanese Patent Application Laid-Open No. S63-25612. In FIG. 17, an alternate long and short dash line 1502 denotes an aberration when the optical element is not inserted to the optical path. A chain line 1501 denotes an aberration when the optical element is inserted to the optical path, and IP denotes an image plane position. The spherical aberration degrades the quality of an obtained image. In addition, if a best focus position (1503, 1504), which can be expressed as a position with a minimum root mean square (RMS) of spot diameter in a spot diagram of axial beam, is not within a focal depth (1505, 1506), the insertion and removal of the optical element significantly affects the image, and the obtained image is greatly changed.
If the inserted optical element does not have positive refractive power as in Japanese Patent Application Laid-Open No. 563-25612 and is a parallel plate, the image forming position changes to the over side when the optical element is inserted, and a spherical aberration to the over side occurs. FIG. 18 illustrates a schematic diagram of the spherical aberration when the parallel plate is inserted. In FIG. 18, an alternate long and short dash line 1602 denotes an aberration when the optical element is not inserted to the optical path. A chain line 1601 denotes an aberration when the parallel-plate optical element is inserted to the optical path, and IP denotes an image plane position.
In FIG. 18, to set a paraxial focal point on the image plane relative to a paraxial beam, the paraxial focal point is adjusted by moving the lens unit in an optical axis direction or moving the image plane. As is clear from FIG. 18, of best focus positions 1603 and 1604, the best focus position 1603 when the optical element is inserted is not within the focal depth (1505, 1506), and it can be recognized that the spherical aberration causes a change in the image due to the insertion and removal. Particularly, the image significantly changes in an optical system with a small f-number.